Some power supply circuits, such as light emitting diode (LED) drivers and laser diode drivers, require a current source with high output current. To achieve high efficiency, minimize device size and thermal stress, switching mode DC/DC converters may be utilized to provide a current source, especially in high current applications.
FIG. 1 illustrates a conventional DC/DC converter 10 that may be used as a power supply for providing current to a load, such as a LED or laser diode. The converter 10 may be a synchronous step-down switching regulator including a pair of MOSFETs M1 and M2 controllable by a control circuit 12 to produce a required value of current iLoad in a load 14, such as a LED. The control circuit 12 may include a pulse width modulation (PWM) circuit for producing a PWM control signal, and switching logic driven by the PWM control signal to control switching of the MOSFETs M1 and M2.
When the MOSFET M1 is on, input voltage Vin is applied to inductor L coupled between the MOSFETs and the load 14. The difference between the input and output voltages across the inductor L causes inductor current iL flowing through the inductor L to increase. When the MOSFET M1 is turned off, the input voltage applied to the inductor L is removed. However, since the current in the inductor L does not change instantly, the voltage across the inductor L adjusts to maintain the inductor current. Hence, the inductor current ramps up when the M1 is on and ramps down when the M1 is off. When the MOSFET M2 is on, the inductor current flows from the inductor L to the load 14 and back through the M2. Resistor RSENSE is the inductor current sensing resistor required by the control circuit for loop compensation and over current protection. Output capacitor Co is connected in parallel to the load 14 to bypass the inductor ripple current. Because of this capacitor, the inductor current iL differs from the load current iLoad. Output capacitor Co is connected in parallel to the load 14 to reduce the inductor ripple current.
Load current iLoad flowing through the load 14 may be directly sensed using load current sensing resistor RLoad coupled in series with the load 14. An error amplifier 16 may compare the sense voltage representing the sensed load current iLoad with a reference voltage Vref representing a desired current value to produce an error signal supplied to a compensation input of the control circuit 12. The error amplifier 16 may include an operational amplifier A1 having a non-inverting input supplied with the reference voltage Vref and an inverting input connected via resistor R1 to a node between the load 14 and the load current sensing resistor RLoad. An RC circuit composed of capacitor C1 and resistor R2 may be coupled between the inverting input and the output of the operational amplifier A1. In response to an error signal at the output of the error amplifier 16, the control circuit 12 controls switching of MOSFETs M1 and M2 to produce a desired value of load current.
However, the load resistor RLoad used for sensing the load current causes significant power loss. Moreover, for high current applications, this current control method not only reduces power conversion efficiency but also increases the supply thermal stress.
Another conventional current control method illustrated in FIG. 2 uses sensing of the inductor current iL to control the load current. A step-down converter 20 comprises MOSFETS M1 and M2, inductor L and output capacitor Co connected in the same manner as in the converter in FIG. 1. To avoid using the load resistor RLoad and reduce power loss, the converter 20 includes a current sensing circuit that senses the inductor current iL. The current sensing circuit may include a differential amplifier A2 having its inputs connected across terminals of sense resistor Rsense coupled in series with the inductor L. The output signal of the differential amplifier A2 is supplied to the error amplifier 16 for comparison with the reference voltage Vref. The error signal produced by the error amplifier 16 is supplied to the control circuit 12 to control switching of the MOSFETs.
However, the control circuit 12 does not directly control the load current iLoad. Instead, the inductor current is controlled. Therefore, the dynamic tracking performance of the converter 20 is compromised. For applications that require a fast load current transient response, such as LED true color dimming control, sensing of the inductor current results in poor load dynamic performance.
FIGS. 3a and 3b illustrate waveforms of various signals in circuits shown in FIGS. 1 and 2, respectively. In particular, FIGS. 3a and 3b show respective waveforms of reference signal Vref, load current iLoad in a LED load and inductor current iL. Ideally, the load current iLoad should accurately follow the reference signal Vref. Due to the control loop delay, the rising and falling edges of the load current pulse are delayed with respect to the corresponding edges of the reference signal. The direct load current sensing illustrated in FIG. 1 provides acceptable waveform of the load current iLoad. However, the waveform of the load current iLoad controlled using the inductor current sensing method in FIG. 2 is not acceptable because the inductor current iL, instead of the load current iLoad, is used to track the reference signal Vref. Therefore, the current control scheme illustrated in FIG. 2 is much slower than the current control scheme in FIG. 1, and cannot be used for applications requiring a fast dynamic response.
Hence, there is a need for a current sensing scheme that would enable the current control system to improve its dynamic performance compared to a conventional control system with inductor current sensing, such as shown in FIG. 2, and would not have power loss disadvantages of a direct load current sensing scheme.